This invention relates to fluid flow measurement and it relates especially, though not exclusively, to the measurement of liquid flow in pipes by means of so-called point-measuring devices.
The preferred technique for measuring the flow of fluids through pipes utilises a full-bore flowmeter, such as an electromagnetic flowmeter, to take direct readings of the desired parameters. Frequently, however, practical constraints associated with various pipework installations militate against the use of such flowmeters for in-situ measurements.
In these circumstances, it is usual to employ a so-called point-measuring device which measures the flow velocity at a particular location within the pipe. In practice, the measurement relates to a small area of the cross-section of the pipe, rather than a point, and references herein to measurements made at or relating to points should be construed accordingly. In principle, measurements taken at one location can be used to calculate the volumetric flow of fluid through the pipe. This calculation, however, demands knowledge of a factor (known as the profile factor) which relates the measured (point) velocity to the mean velocity of fluid in the pipe as a whole. Whilst, in the case of a long straight pipe, the distribution of flow follows well established rules, rendering the profile factor readily predictable, most installed pipework layouts do not conform to this ideal, so the standard predictions cannot, in many cases, be used.
It thus becomes necessary to use the point-measuring device to take a number of readings over a cross section of the pipe, to establish the profile factor by measurement.
In practice, various factors can adversely influence the measurements and present methods based on this principle exhibit drawbacks in being unable to compensate adequately for the effects of such factors and/or to warn the user that some or all of the measurements may be flawed or contaminated to a degree that renders them unreliable.
GB-A-2045948 discloses a method and system for conducting multiple velocity measurements transfer of a flow stream cross section of known shape and area to determine flow therethrough.
U.S. Pat. No. 5,186,150 discloses flow rate measuring method and apparatus for determining a flow rate of a fluid through a pipe by using a fuzzy logic system.
It is an object of this invention to provide a method of measuring fluid flow in which the aforementioned drawback are at least reduced.
According to the invention there is provided a method of measuring fluid flow through a pipe including the steps of:
(a) obtaining a plurality of measurements indicative of the flow velocity a t each of a number of locations distributed over a cross-section of the pipe;
(b) deriving from at least a first subset of the measurements an evaluation of flow velocity distribution over the cross-section of the pipe;
(c) deriving from the change over time of at least a second subset of the measurements, an estimate of the stability of th e measured flow velocity, and
(d) providing an indication based upon said estimate of stability, of the reliability of said flow velocity distribution.
Step (b) of deriving an evaluation of flow velocity may comprise averaging some at least of the plurality of measurements.
In a closely related aspect, there is provided a method comprising:
(a) using a measuring device to take over a period of known duration a plurality of measurements indicative of the flow velocity at each of a number of locations distributed over a cross section of the pipe;
(b) averaging some at least of each plurality of measurements to provide averaged measurements;
(c) utilising the averaged measurements to derive an evaluation of the distribution of flow velocity over the cross-section of the pipe;
(d) deriving from at least some of said measurements of flow velocity an indication of the stability of the measured flow velocity; and
(e) providing an indication, based upon said indication of stability, of the reliability of said evaluation.
Preferably, said duration is between 20 and 120 seconds, and the measurements of flow velocity are taken in sequence at each of said locations by means of an electromagnetic insertion probe such as that known by our trade mark xe2x80x9cAquaProbexe2x80x9d. Such probes measure the flow velocity over a small area around their tip and are thus considered to be point-measuring devices.
The invention also extends to corresponding apparatus.